1. Field of the Invention
This invention relates to an integrally molded article of polypropylene resin and silicone rubber which is useful in the areas of electric equipment, electronic equipment, automobiles, precision machines and the like and a method for preparing the same.
2. Prior Art
A number of methods have been proposed for providing a bond between addition curing type silicone rubber and organic resins. It is known, for example, to form a bond by applying a primer to a surface of molded resin, applying uncured silicone rubber material thereto and curing the silicone rubber to the resin or by curing self-adhesive silicone rubber compositions directly to molded resin. For the self-adhesive silicone rubber compositions, a number of proposals have been made on their tackifier component.
As another approach, it is known from Japanese Patent Publication (JP-B) No. 34311/1990 to add an organohydrogenpolysiloxane containing at least 30 mol % of hydrogen atoms directly attached to silicon atoms to an organic resin so that the resin is bondable with addition curing type silicone rubber. JP-B 45292/1988 discloses integration by physically fitting silicone rubber within molded organic resin. In Japanese Patent Application Kokai (JP-A) No. 193843/1988, a compound having an aliphatic unsaturated group and a hydrolyzable group attached to a silicon atom is grafted to an olefin resin and silicone rubber is bonded and integrated with the grafted olefin resin. Furthermore, as we previously proposed, a thermoplastic resin can be bonded and integrated to a silicone rubber composition when a compound having an unsaturated group and a hydrogen atom directly attached to a silicon atom is added to the resin (U.S. Ser. No 07/965,303 and EP 0540259 A1).
However, several problems arise with these prior art methods for integrating silicone rubber and organic resin into a one-piece article. The primer method is cumbersome in that a once molded resin shape must be taken out of the mold before the primer can be applied thereto.
Where a self-adhesive addition curing type silicone rubber material is applied and cured to a resin preform, the resulting bond strength is insufficient for use as a one-piece member if the resin is a polypropylene resin.
Where organohydrogenpolysiloxane is added to olefin resin, the properties of the resin itself can be altered thereby, preventing the resin from exerting its own properties. The physical engagement method leaves a possibility that the two segments be disengaged by physical forces. The use of an olefin resin having grafted thereto a compound having an aliphatic unsaturated group and a hydrolyzable group attached to a silicon atom cannot dispense with a primer when it should be joined to addition curing type silicone rubber.
In these years, silicone rubber has found a spreading use in the fields of electric and electronic equipment and automobiles because it is recognized highly reliable in the aspects of heat resistance, weatherability and electrical properties. To meet such a demand, there is a desire to have integral moldings in which thermosetting resin, especially polypropylene resin and silicone rubber are firmly united or bonded together.